Seismic prospecting with optimum geophone coverage



De. 23, 1952 J. F. BAYHI ET AL 2,523,113

sEIsMIc PROSPECTING WITH OPTIMUM GEoPHoNE covERAGE Filed Nov. 1 1951 2 SHEETS- SHEET 1 To Qadofwme (i2 (Ampmgls To Q Eciofzvme (AMDM F l als To faciormme AMPLI F11-:ls P

. l v f2? s? l-5);( @,j 7E( N- 1 15 Con/1511111.16 AND ISOLA-rm@ @muws HHH Joseph F1. aghi.

Uohn. T @3414er nvenbor's Patented Dec. 23, 1952 SEIASMIC PROSPECTING WITH OPTIMUM GEOPHONE COVERAGE Joseph F. Bayhi and John T. Baker, Tulsa, Okla.,

assignors to Standard Oil Development Company, a. corporation of Delaware Application November 1, 1951:, Serial No. 254,254l

3 Claims. Y 1

This invention concerns improvements in the art of seismic prospecting and is directed particularly to an electronic circuit and geophone arrangement wherein the effect of using an overlapping pattern of geophones in the seismic spread is obtained without actual use of such overlapping.

A method commonly employed in searching for areas'I likely to contain oil or other mineral deposits is that known as seismic prospecting wherein a seismic disturbance is initiated at a selected'point in or on the earths surface as, for example, by detonating an explosive charge in a shot hole, which causes seismic waves to travel 'through the earth and to be reflected from various substrata, the upward traveling reflected waves being detected at a number of points spread out in a desired pattern from the point of the initial seismic disturbance. Sensitive pickups called seismic detectors, seismometers or geophones, are arranged at the detection points to translate the detected motion into electrical impulses which after suitable amplication are recorded on a seismograph. The records may be in the form of waves or traces representative of the seismic waves that have been picked up by the individual geophones or they may be in the form of ,variable density or variable area records and in each case will be plotted as a function oi time along the record, suitable timing marks being simultaneously made on the record so that when the same is later examined it will be possible to determine the length of time required for the arrival of thedetected waves at any particular one of the detection points. From other data obtained Vin the area being studied, for example, seismic wave velocities in various earth layers, it is then possible to estimate the depth of the various' substrata.

Although it is theoretically possible to time the arrival of a reiiected seismic wave by the use of a single geophone and recording device, in practice it is usually diiiicult and sometimes impossible to pick out indicated reiiected waves from a number tof otherj earth vibrationsv that are detected andl recorded at the same time. Therefore the usualpractice is to employ a plurality of seismo'meters'fspread over a considerable distance along the earths surface in a selected pattern as justA described and to make aplurality of traces in vside`byside relation on a single chart for pur, poses'of comparison, sincefa reflection from awell deiin'edl 'stratum'will' appear on the record as a waveform,V of increasediamplitudeon all ofthe traces iii-some 'deiinte timerelation, thus permit# ting the reflection to be lined up on the record.

It has been found that when diiculty is en-v countered in obtaining suitable reflections on the record in some prospecting areas significant improvements in the ratio of usable to spurious signals or, in other words, in the ratio of reiiection to non-reflection energy can often be obtained by using a plurality of geophones at each detec-` tion station all tied to one trace on the record. Some improvement is' also obtained by using greater surface coverage for the geophones connected to each trace. However, it is desirable to avoid excessive time step-out between the arrival of related events on adjacent traces when the individual trace coverage is made greateryhence the separation between'detection stations should remain essentially the same. In order to do this it is frequently necessary to have the geophones tied toene trace overlap those of adjacent traces;

It is an object of the present invention to fur'- nish a circuit arrangement whereby the desirable effect of multiple geophone patterns is obtained without requiring that any overlappingof geo-#- phones occur. This is accomplished byproviding an electronic circuit to take the place of the overlapping geophones, which reduces the time required to lay out the geophones at each detection station.

Thejnature and pobjects of the invention and the manner in which itis utilized will be more clearlyappreciated from the ensuing description,

particularly when taken in conjunction with the accompanying drawing in which: j Figure 1 isa schematic diagram of a conventional multiple geophone spread for six recording In Figure 1 a conventional multiple geophone spread for six recording channels is shown, the' geophones Il being placed upon or embedded in` the surface of the ground I0 and evenly spacedv with a separation, Vfor example, oi 3Q feet'. Suit able connection ,i'sjinadevbetween each group of geophones and recording ampliers through a In Figure' 2 there sshown in plan viewa more desirable geophone spread covering a distance of 810 feet as compared to 690 feet with the spread shown in Figure 1 while preserving the same separation of 30 ft. between adjacent geophones. It will be noted that in order to accomplish this, overlapping between adjacent groups of geophones is necessary. Thus the last four of the geophones in group IIa overlap the rst four geophones in group I Ib, and so on. The overlapping geophones in each group are marked with crosses. If these four overlapping geophones in each group are placed side by side with those of the adjacent group with which they overlap the signals from the two sets of geophones will be almost identical. In the present invention the benefits of such an overlapping can be obtained by omitting the overlapping geophones, i. e. those marked with crosses, and replacing them with an electronic isolating and overlapping circuit as shown schematically in Fig. 3. Here seven groups of geophones with four units in each group are employed and the same spacing of 30 ft. between adjacent geophones is used. The signals from the geophones are fed through cable I2 to an isolating and overlapping circuit I5 and from thence to the usual recording amplifiers, six traces being made from the seven groups of geophones. Thus the same coverage of 810 ft. is made with 28 geophones rather than with the 48 geophones of Figure 2, which reduces the geophone setup time by a factor of 1.7.

In Fig. 4 is presented a diagram of a suitable electronic circuit constituting the isolating and overlapping arrangement I5 of Fig. 3. This circuit consists of a number of primary and auxiliary cathode follower stages, each primary cathodeA follower stage having its cathode circuit common to that of the corresponding auxiliary cathode follower stage. The output signal from a first group of geophones is fed to the first primary cathode stage, the output signal of a second group of geophones is fed to the second primary cathode stage and to the first auxiliary cathode stage, and so on, the output signal of the final group of geophones being fed to the final auxiliary stage only. The outputs from each primary cathode stage are fed to a channel of a recording seismograph. Since the number of primary stages is one less than the number of geophone groups the number of traces will be one less than the number of geophone groups. Thus, as mentioned above, with seven geophone groups there will be six recorded traces.

Referring now in detail to Fig. 4 it will be seen that the output of geophone group I Iis fed through input transformer 2l to a cathode follower stage comprising Vacuum tube 22, cathode resistor 26 and isolating capacitor 21 to output terminals 28 and 29, which may be connected to one channel of a seismograph recording amplifier. Likewise, signals from geophone group 2 are fed through transformer 3l to a cathode follower stage comprising vacuum tube 32, cathode resistor 36 and isolating capacitor 31 to output terminals 38 and 39, which may be connected to another channel of a recording amplifier. At the same time the signal from geophone group 2 is fed to terminals 28 and 29 through a cathode follower stage whose cathode circuit is common to that of the stage receiving signals from geophone group I, this cathode follower stage comprising vacuum tube 23 and cathode resistor 26. Thus the signal appearing at terminals 28 and 29 is made up of components from the signals of geophone groups I and 2. In the same manner the 4 total signal appearing at terminals 38 and 39 is made up of components from the outputs of geophone groups 2 and 3 and the total signal at terminals 48 and 49 is made up of components from geophone groups 3 and 4. Using the same terminology as above, the cathode follower stage comprising vacuum tube 22 may be designated the first primary stage and the stage comprising tube 23 may be designated the first auxiliary stage. Signals from the final group of geophones will be fed to a final auxiliary stage but not directly to a final primary stage. A simple case can be illustrated by Fig. 4. If there are only three groups of geophones, the signals from geophone group 3 will be fed to Vacuum tube 33 but vacuum tube 42 will not be used. Thus the signals from the .three geophone groups will be recorded on only two traces, these being connected into terminals 28 and 29 and 38 and 39. In other words the number of traces will be one less than the number of geophone groups, as already stated.

The isolating and overlapping electronic circuit of this invention utilizes cathode coupled stages to combine the outputs of two or more adjacent geophone groups and feed the combined outputs into a single recording channel. It should be noted, however, that each geophone group is electrically isolated from adjacent groups by the vacuum tubes so that, for example, a signal from geophone group I is not mixed with that from geophone group 2 by means of resistor or transformer networks. Furthermore the signal from geophone group I that contributes to the character and magnitude of the total signal appearing at terminals 28 and 29 plays no part whatever in forming the signal appearing at terminals 38 and 39. Likewise the signal from geophone group 2 contributes to the character and magnitude of the total signal appearing at the output terminals 28 and 29, and 38 and 39 but plays no part in forming the signal at terminals 48 and 49. It is evident that the recorded trace of the recording channel fed by terminals 28 and 29 will have in common with the recorded trace of channel fed by terminals 38 and 39 only that signal which is the output of geophone group 2. Hence only a portion of the character and magnitude of the resultant signal on each channel is common to that of adjacent channels. Thus the circuit of the present invention is quite different from the conventional geophone mixing circuits sometimes referred to as CMI circuits wherein a certain percentage of the entire signal from each recording channel is combined with the signal from an adjacent channel to form the signal for a certain seismograph trace. Therefore a fundamental disadvantage associated with conventional seismograph mixing or compositing wherein forcing of reiiections occurs is avoided in the circuit of this invention.

With the arrangement just described it will be noted that a 50% overlap between adjacent traces is obtained. A 662/3% overlap can be obtained by use of additional vacuum tubes in the circuit as indicated by the portion of the circuit shown in phantom in Figure 4. Thus the signal from geophone group 3 will be fed through vacuum -tube 24 to combine with the signals appearing across terminals 28 and 29. Th total signal on the recording channel connected to terminals 28 and 29 will now be the resultant of the character and magnitudes of the signals from geophone groups I, 2 and 3. In the same manner a third vacuum tube in each of the succeeding stations associated with geophone group t, 5, etc. may be used to feed signals to terminals SS and 39, 63 and 4S, and so on. Conveniently the circuit could be so arranged that either 59% or 662/3% overlap could be obtained merely by cutting vacuum tube 2li and related vacuum tubes in or out of the circuit, e. g. by supplying or not supplying heater current to these vacuum tubes.

It will be apparent to persons skilled in the art that many modications of this invention are possible without departing from its scope. it is therefore intended that the invention not be limited to the specific examples presented. The scope of the invention is defined by the following claims.

What is claimed is:

1. Apparatus for recording detected seismic waves at a plurality yof spaced detection points adjacent the earths surface comprising at least one seismic detector at each of said detection points, an electronic circuit adapted to combine characteristics of the signals received at each detection point with signals received at adjacent detection points, said electronic circuit comprising a plurality of primary cathode follower stages, one less in number than the number of said detection points, a plurality of auxiliary cathode follower stages equal in number to said primary cathode follower stages, the first of said primary stages having a cathode circuit common l to that of the first of said auxiliary stages and each succeeding primary stage having a cathode circuit common to that of each succeeding auxiliary stage, means connecting the outputs of the detectors at a rst detection station to the input of the rst primary cathode follower stage, means connecting the outputs of the detectors at the nal detection station to the input of the nal auxiliary-stage, and additional means con necting the outputs of the detectors at each intermediate station to the inputs of succeeding intermediate primary stages and to the inputs of the corresponding immediately preceding auxiliary stages. v

2. Apparatus for recording detected seismic waves at a plurality of spaced detection points adjacent the earths surface comprising a plurality of seismic detectors arranged in spaced relation at each of said detection points, an electronic circuit adapted to combine characteristics of the signals received at each detection point with signals received at adjacent detection points, said electronic circuit comprising a rst primary cathode follower stage, means connecting the outputs of the detectors at a first detection station with the input of said rst primary cathode follower stage, a rst auxiliary cathode follower stage having a cathode circuit common to that of said first primary cathode follower stage, a second primary cathode follower stage, means connecting the outputs of the seismic detectors at a second detection station with the inputs of said second primary cathode follower stage and of said rst auxiliary cathode follower stage, a second auxiliary cathode follower stage having a cathode circuit common to that of said second primary cathode follower stage, an additional primary cathode follower stage and an additional auxiliary cathode follower stage with a common cath-ode circuit for each additional detection station less one, means connecting the detectors at each of said additional detection stations, less one, with the inputs to the corresponding primary and intermediately preceding auxiliary cathode follower stages in the manner aforesaid, and means connecting the detectors at the final detection station with the input to the nal auxiliary cathode follower stage.

3. Apparatus according to claim 2 including a third set of cathode follower stages, the same being less in number by two than the number of detection stations, the number of detection stations being at least three, the first of said third set of cathode follower stages having a cathode circuit common to that of said rst primary cathode follower stage and said iirst auxiliary cathode follower stage, the means connecting the inputs of the seismic detectors at a third detection station with the input of the second auxiliary stage also connecting the same with the input of the rst of said third set of cathode follower stages, the additional connecting the outputs of the seismic detectors at additional detector stations with the respective additional auxiliary stages likewise connecting said outputs seriatim with the inputs of the respective remaining members of said third set of cathode follower stages.

JOSEPH F. BAYHI. JOHN T. BAKER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,473,469 Dahm June 14, 1949 2,509,651 Olson May 30, 1950 

